The invention relates to dinitromethane, more specifically to a method of preparing dinitromethane in the form of its salt with an organic or inorganic cation.
Free dinitromethane is an instable yellowish oil which decomposes even at room temperature. Salts of dinitromethane, especially alkali metal salts, are stable compounds which have an important application as intermediate product and starting material for production of explosives and propellants containing gem-dinitro groups. Salts of dinitromethane can also be used as oxidiser in propellant compositions and pyrotechnic charges and in this application they have the advantage of being an oxidiser free from chlorine.
A method of synthesising alkali metal salts of dinitromethane is disclosed in U.S. Pat. No. 4,233,250. This prior-art method is begun by nitration of cyanooximino acetate to form methylcyano dinitroacetate which is then hydrolysed to form dinitroacetonitrile, which in turn is neutralised with an alkali metal hydroxide to form a corresponding alkali metal salt of cyanodinitromethide. The cyanodinitromethide is then saponified with an alkali metal hydroxide in aqueous solution to obtain a corresponding alkali metal salt of dinitromethane.
An object of the present invention is to provide a simpler and less expensive method for preparing alkali metal salts of dinitromethane and other organic as well as inorganic dinitromethane salts.
This is achieved by a method as defined in the claims.
According to the invention, a heterocyclic compound of the general formula 
and tautomers thereof, where X1 and X2 are the same or different and selected from a group consisting of xe2x95x90O, xe2x95x90S, xe2x80x94OH, xe2x80x94SH, halogen, xe2x95x90Hxe2x80x94OH, xe2x80x94NH2, xe2x80x94NHxe2x80x94R where Rxe2x95x90alkyl and X3 is selected from a group consisting of xe2x80x94H, alkyl, xe2x80x94NO2 , xe2x95x90O, xe2x95x90S, xe2x80x94OH, xe2x80x94SH, xe2x80x94N3, xe2x80x94CN, xe2x80x94CNO, xe2x80x94NCO, xe2x80x94CHO, xe2x80x94COOH, xe2x80x94COORxe2x80x2, xe2x80x94C(O)xe2x80x94Rxe2x80x2, xe2x80x94C(S)xe2x80x94Rxe2x80x2, xe2x80x94C(O)xe2x80x94Sxe2x80x94Rxe2x80x2, xe2x80x94C(S)xe2x80x94Sxe2x80x94Rxe2x80x2, xe2x80x94C(O)xe2x80x94N(Rxe2x80x2)xe2x80x94Rxe2x80x3 where Rxe2x80x2 and Rxe2x80x3 are alkyl or aryl,
is nitrated with a nitrating agent to form a gem-dinitro compound containing the structural element 
in the heterocyclic ring structure;
said gem-dinitro compound is hydrolysed in an aqueous medium to split off dinitromethane, and
said dinitromethane is neutralised with a neutralising agent to form a corresponding dinitromethane salt.
Suitable starting compounds are e.g.
barbituric acid

4,6-dihydroxy-2-methylpynmidine

xe2x80x83and
1,2,4-triazol-5-on

In nitration use is made of a conventional nitrating system consisting of nitric acid or nitric acid and/or nitrate salts in combination with some other acid. In the first place, nitric acid/sulphuric acid (HNO3/H2SO4) is preferred, but other conceivable nitrating acids are nitric acid/perchioric acid (HNO3/HCIO4), nitric acid/phosphoric acid (HNO3/H3PO4), nitric acid/diphosphorus pentoxide (HNO3/P2O5), nitric acid/acetic acid, nitric acid/acetic anhydride, nitric acid/trifluoroacetic acid and nitric acid/trifluoroacetic anhydride.
The nitration is carried out at a low temperature, such as xe2x88x9220-+50xc2x0 C., and with a moderate acidity of the nitrating system, e.g. 70-100% nitric acid and 70-100% sulphuric acid. Under these circumstances there forms in the reaction mixture a product which contains the structural element [4]. Depending on starting compound and used nitrating acid, temperature and acidity may need be adjusted for an optimal yield of the product. Particularly good results have been achieved with nitric acid (90-100%) and sulphuric acid (90-100%), a temperature of 10-50xc2x0 C. and a molar ratio of nitric acid to substrate of 2.0-6.0:1, preferably 2.0-3.0:1.
Depending on whether the formed gem-dinitro product [4] is soluble in the nitrating mixture or not, the further process is carried out in slightly different manners.
When the gem-dinitro product [4] forms a precipitate in the nitrating mixture, the product can easily be separated, dissolved in an aqueous medium and hydrolysed with water, optionally after adding a suitable base. In the hydrolysis, dinitromethane is split off, which with an added neutralising agent gives a corresponding dinitromethane salt. For certain starting compounds the product [4] hydrolyses spontaneously and splits off dinitromethane when it is dissolved in water. A second precipitate may then form, which derives from the remaining part of the product [4]. This second precipitate is separated from the aqueous solution before the neutralising agent is added. When adding a base, salts of the dinitromethane ion can be obtained either as dissolved salt or as a solid precipitate depending on the solubility of the formed salts in water. When a precipitate forms, it may easily be separated and, if necessary, be purified by recrystallisation.
Water-soluble dinitromethane salts can, for instance, be separated from the other salts in the mixture by contacting the mixture with an adsorbing agent that adsorbs the dinitromethane salt. Suitable adsorbing agents are activated carbon, silica gel and zeolites. Dinitromethane salts have a higher affinity to these adsorbing agents than, for example, nitrates, sulphates etc. in the neutralised aqueous solution. The adsorption material is suitably arranged in a column through which the aqueous solution is conducted. The dinitromethane salt can then be eluted with hot water and/or a polar organic solvent, such as acetone, 2-propanol, acetonitrile, methanol, ethanol, and be crystallised by evaporation of the eluant.
Free dinitromethane in aqueous solution can also be extracted with a suitable organic solvent, for instance ethyl ether, dichloromethane etc, whereupon the neutralising agent is added to the obtained organic phase for precipitation of a corresponding dinitromethane salt.
In the cases where the product [4] does not precipitate as a solid compound but remains dissolved, the entire nitrating mixture can be diluted with water and, optionally, neutralised to hydrolyse and cleave the product [4]. Subsequently the solution can be acidified with a suitable acid to transform all dinitromethane into the free form, whereupon the dinitromethane is extracted with an organic solvent. The resulting organic phase is then neutralised with a neutralising agent and a corresponding dinitromethane salt precipitates.
An organic or inorganic base is used as the neutralising agent. Suitable neutralising agents that can be used are ammonia NH3, hydrazine N2H4, a primary amine RNH2, a secondary amine RRxe2x80x2NH or a salt AX, where A is a metal ion or a nitrogen-containing cation and X is a fluoride, chloride, hydroxyl, carbonate,alkoxide or carbonyl ion. R and Rxe2x80x2 in the amines can be the same or different and constitute alkyl groups having 1-6 carbon atoms.
The respective neutralising reactions can be illustrated by means of the following reaction formulae:
H2C(NO2)2+NH3xe2x86x92NH4+xe2x88x92CH(NO2)2
H2C(NO2)2+N2H4xe2x86x92N2H5+xe2x88x92CH(NO2)2
xe2x80x83H2C(NO2)2+RNH2xe2x86x92RNH3+xe2x88x92CH(NO2)2
H2C(NO2)2+RRxe2x80x2NHxe2x86x92RRxe2x80x2NH2+xe2x88x92CH(NO2)2
H2C(NO2)2+AXxe2x86x92A+xe2x88x92CH(NO2)2
Metal ions that may constitute A in the salt AX include mono-, di- or trivalent metal ions which form soluble salts with fluoride, chloride, hydroxyl, carbonate, alkoxide or carbonyl ions. Typical metals include alkali metals Li, Na, K, Rb and Cs; alkaline earth metals Be, Ca, Ba, Sr and Mg; group Ib metals Cu, Ag and Au; group IIb metals Zn, Cd and Hg; group III metals Al, Sc, Y, Ga, In and elements 57-71 of the Lanthanoide series; group IV metals Ti, Zr, Hf, Ge and Sn; group V metals V, Nb, and Ta; group VI metals Cr, Mo and W; group VIIa metals Mn, Tc and Re; and group VIII metals Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and Pt. Preferred metals are Li, Na, K, Be and Mg, especially K.
When A is a nitrogen-containing cation, it may consist of an ion of the formula (Rxe2x80x3kHmNn)+z, where n=1-8, k=0 to 2+n+z, z=1 to n, m=2+n+z-k and Rxe2x80x3 is a straight or branched alkyl having 1-6 carbon atoms. Examples of such ions include NH4+, CH3NH3+, (CH3)2NH2+, (CH3)3NH+, (CH3)4N+, C2H5NH3+,(C2H5)2NH2+, (C2H5)3NH+, (C2H5)4N+, (C2H5)(CH3)NH2+, (C2H5)(CH3)2NH+,(C2H 5)2(CH3)2N+,(C3H7)4N+,(C4H9)4N+, N2H5+, CH3N2H4+, (CH3)2N2H3+, (CH3)3N2H2 +,(CH3)4N2H+, (CH3)5N2+,etc.
A may also consist of a cubane-1,4bis-ammonium ion, cubane 1,2,4,7-tetraammonium ion; cubane-1,3,5,7-tetraammonium ion; cubane-1,2,3,4-tetraammonium ion; cubane-1,2,3,4,7-pentaammonium ion; cubane-1,2,4,6,8-pentaammonium ion; guanidinium (C(NH2)3+); triaminoguanidinium (C(N2H3)3+); guanylurea ion (H2NC(=NH)NHCONH3+); nitronium (NO2+); nitrosonium (NO+) or a 1-10000 nitrogen polymer of ethyleneimine.